CVTs are well known transmission mechanisms that can change trough an infinite number of gear ratios between upper and lower limits. Toroidal CVTs, which are also well known, include a disk and roller arrangement that transmits power between the disks, wherein one disk is the input and the other disk is the output. Such a transmission is used when transmission ratios have to be finely adjusted.
However, the ratio range required on a vehicle is often such that the size required for a CVT to cover the entire ratio range would be so large that it would be impractical to position it in some vehicle.